1. Technical Field
The present invention relates to fabrication of exhaust gas catalytic conversion systems; more specifically it relates to a structure arising from a particular method of fabrication which enables inductive heating to activate exhaust gas purifying catalyst. The structure is applicable to the catalytic conversion system in connection with an internal combustion engine driven motor vehicle.
2. Description of the Background
Catalytic convertors for vehicles with internal combustion engines are effective to treat noxious components such as HC, CO, and NO.sub.x contained in gases exhausting from the engine by breaking them down into less environmentally contaminating derivatives prior to their exit into the atmosphere.
As regulations imposed on manufacturers governing the levels of such pollutants in vehicle exhaust emission which are permissible have become more and more stringent, the demand for improvements in exhaust gas catalytic purification systems has increased.
One type of such systems incorporates a catalyst applied as a layer onto the surface of exhaust system carrier materials including a catalytic converter support network as well as exhaust conduits. A general manufacturing requirement therein is to form the catalyst layer such that it is not liable to peel away from the base material.
Accordingly, the catalyst layer can be formed on the surface of the catalyst carrier via an intermediate layer. One example is described in Japanese Patent Laid-Open No. 71898/1982, wherein oxide whiskers are grown on the surface of a metallic carrier, composed of a ferric stainless steel alloy containing aluminum, to form a catalytic bonding intermediate layer, onto which a catalyst layer is formed. The bonding layer whiskers superficially formed on the surface of the metallic carrier firmly adhere the catalyst layer. Furthermore, the bonding layer, being superficially formed on the metallic carrier though a chemical reaction therewith, is adhered to it very firmly.
A significant drawback of this method of bonding a catalyst to base material is that it takes a considerable amount of time to form the bonding layer.
Since catalytic materials are effective at the high temperatures of the exhaust gas itself in breaking down the noxious components thereof, purification performance during warm-up to normal operating temperature following engine cold start is poor, and hydrocarbons and other untreated pollutants temporarily are exhausted into the atmosphere. The cumulative effect from the multitude of motor vehicles is significant enough to warrant treatment.
Recent attempts made to enhance the catalytic materials themselves to be effective for exhaust gas purification at the lower temperatures during engine warm-up have met with limited success.
Accordingly, means to accelerate the heating of the exhaust-treating catalyst in advance of engine warm-up have been proposed, as disclosed for example in Japanese Utility Model Application Laid-Open No. 36324/1974, which describes the installation of an electric heater in close proximity to the end of a monolithic-structured catalyst and upstream with respect to exhausting gas flow.
Therein, heat energy produced by electric current through heater wire spaced apart from the catalyst-bearing support heats gas at the mouth of the catalytic converter, and the heated gas in turn heats the catalyst as it enters the monolithic structure. The heated gas superficially contacting the catalyst does not serve as an efficient heat transfer medium to effectively heat the catalyst, since its temperature drops considerably as a consequence of mixing with surrounding gas. The amount of electrical current supplied to the heater necessary to overcome this problem is too great to be practical for apparatus in automobile installations.
Another proposal is in Japanese Utility Model Laid-Open No. 67609/1988. Specifically therein, a conductive metallic base material is used as a catalyst carrier, and the carrier is electrically energized in order to heat the exhaust-purifying catalyst to a high temperature.
This way of preheating the catalyst by electrically energizing the metallic carrier is not satisfactory in that considerable time is taken to heat the metallic carrier to sufficiently high temperature, tending to nullify the purpose of catalytic preheating. Moreover, since a large amount of electricity is drawn from the battery by this preheating system immediately after the engine is started, a heavy burden is imposed on it.
The metal support carrying the catalyst and as such forming a monolithic construction is integral in the electric heating circuit, which is liable to bring about the drawback of localized heating of the support, such that considerable differences in temperature will exist from one portion thereof to another. Consequently, different rates of expansion arise, causing separation of the catalyst material from the support.
Japanese Utility Model Application Laid-Open No. 124412/1974 proposes a monolithic catalyst assembly which comprises a platinum catalyst carried in a monolithic structure on a support of silicon carbide which serves as a heating element, wherein the catalyst may be heated upon application of an electric current to the heating element.
Similarly. Japanese Patent Application Laid-Open No. 295184/1991 proposes a honeycomb-structured support as a catalyst carrier, formed as a product of powder metallurgy, to which an electric current is directly applied in order to heat it.
Wherein the catalyst support of the monolithic construction is designed to function itself as a heating element, it is essential that the support, in composing an electric circuit, have a controlled resistance, whether the support is made of silicon carbide or a metal, or formed from a product of powder metallurgy. This type of construction, however, not only adds to the cost of the catalytic purifier apparatus, but also presents a problem in reliability, since an undesirable change in resistance is likely to occur during cycles of actual operation as a result of, for example, the separation of metal-to-metal joints wherein the support is formed from a metal foil, or as a result of partial cracking of a sintered support. Moreover, production of a reliable and satisfactory catalytic purifier construction is hindered by the difficulty of adequately forming electrode portions on the foregoing support.